Current variable speed fan drive systems used in work vehicles such as agricultural tractors typically use a viscous drive on the system cooling fan. The viscous fan drive is intended to modulate fan speed and thus air flow, with the resulting power consumption to meet the cooling needs of the vehicle in an optimal manner in an effort to conserve power and fuel. Current drives are cost effective, yet are sized to achieve the lowest cost thereby resulting in durability issues. Additionally, the fans are not as efficient as desired, especially at the mid-point of the modulation range where the desired fan speed for effective cooling resides for a very high percentage of the time in typical work vehicle applications. The problem is that at mid-range fan speeds the combination of moderate speed and moderate torque combine to produce large heat loads that both consume power and generate heat thereby deteriorating the fan drive and thus its reliability. Known fan drives are generally not allowed to modulate in this area and are moved to a higher speed to avoid excess slippage. The result is to drive the fan and its power requirements higher than needed.
Accordingly, there is a clear need in the art for a variable speed fan drive which operates at a much higher level of efficiency and results in less power loss by better optimizing the speed of the fan to the requirements of the cooling system so as to reduce engine costs, improve fuel efficiency and reliability.